1. Field of the Invention
The present invention relates to a 4-port wavelength selective router that routes the counter-propagating signals while suppressing relative intensity noise induced by the multiple back reflections in the bidirectional transmission systems and networks.
2. Description of the Related Art
The bidirectional signal transmission over a single fiber is advantageous compared with the unidirectional signal transmission. It enables full-duplex communications between two nodes with a single strand of optical fiber. It also alleviates the nonlinear effects of optical fiber and thereby enables to achieve higher spectral efficiency.
However, the bidirectional transmission systems suffer from the optical back reflections. In bidirectional transmission systems, the counter-propagating signals are usually allocated at different wavelengths. Thus, we can suppress the reflected light by using optical filters at the receiver. However, we cannot remove the multiple reflected lights in such a way and the multiple reflected lights causes a relative intensity noise. The magnitude of the relative intensity noise is proportional to the square of optical amplifier gain. Thus the relative intensity noise limits the maximum available amplifier gains of the bidirectional transmission systems and networks.
FIG. 1 shows a schematic diagram of a conventional wavelength-division multiplexing (WDM) bidirectional transmission system. It also illustrates a relative intensity noise generation path in the bidirectional transmission system. Each node comprises a transmitter (TX) that generates the WDM signal to be transmitted to the other node and a receiver (RX) that receiving the WDM signals transmitted from said the other node. Also an optical circulator (Cir) will be installed at each node to route the receiving and the transmitting signals. Several bidirectional optical amplifiers (BA) are installed in the bidirectional transmission link deployed between two nodes to compensate for the loss of optical fibers (10).
In this WDM bidirectional transmission system, the output wavelengths of the two nodes are different. We can allocate the wavelengths of the counter-propagating signals according to two different methods: band split scheme and wavelength-interleaved scheme. In the band split bidirectional transmission system as shown in FIG. 2, the wavelengths of WDM signals being transmitted in the same direction are contiguous, while the wavelengths of counter-propagating signals are allocated in different wavelength bands. In the wavelength-interleaved bidirectional transmission systems as shown in FIG. 3, the counter-propagating signals are interlaid in wavelength domain.
By allocating the different wavelengths for the optical signals propagating in the opposite directions, we can eliminate the reflected noisy light generated by the simple reflection. In other words, even if the signal propagating in one direction is reflected at the optical fibers (10) or other optical components and then combines with the other direction signal, the reflected light will be eliminated at the receiver (RX) by an optical filter. However, the optical filter installed at the receiver (RX) cannot remove the multiple reflected noisy lights because their wavelengths are same as those of the signal lights.
For an example, a signal reflected at an optical fiber (10) would be amplified at the optical amplifier (BA). If this reflected signal were to be reflected again at another optical fiber, it would be amplified again, and combined with the original signal as shown in FIG. 1. In such a case, the wavelength of the multiple-reflected noisy signal is identical to that of the original signal, and thus would not be removed by the optical filter installed at the receiver (RX). Therefore it is necessary a method to suppress the multiple-reflected light in a bidirectional optical transmission system.
In occasion, it is necessary to receive or transmit selected signals at an intermediate node of the bidirectional transmission link. In such a case the bidirectional WDM transmission systems further comprises add/drop multiplexer (ADM) at the intermediate node. FIG. 4 is a schematic diagram of a WDM bidirectional transmission system further comprising a conventional add/drop multiplexer (ADM) that add/drop signals with specific wavelengths.
The conventional add/drop multiplexer (ADM) comprises a de-multiplexer (D), 2×2 optical switches (Sw) and a multiplexer (M).
In this case, two optical circulators (Cir) are used to separate/combine the counter propagating at the input and the output port of the add/drop multiplexer (ADM). The optical signals transmitted from left to right is first routed to the de-multiplexer by the optical circulator (Cir) and then separated as their wavelengths by the de-multiplexer (D). The 2×2 optical switches (Sw) connected to the output ports of the de-multiplexer establish transmission paths for the demultiplexed signals to be dropped or passed though the add/drop multiplexer (ADM). We can add the same wavelength signals with the dropped signals though the optical switch. The outputs of the optical switches are multiplexed by the multiplexer (M) and enter into another optical circulator. The optical circulator route the signals into the optical fiber.
Here, the relative intensity noise can be generated through the transmission path of the signal passing through the ADM as shown in FIG. 4.
Therefore, a means for suppressing the relative intensity noise should be incorporated with the with the ADM.